The present invention relates to data bus distribution systems and more particularly to multiuser protected optical data bus distribution systems.
Bus or loop distribution techniques are particularly attractive because they allow a number of users to communicate flexibly without the usual intervention of a switching element or a store and forward element. Routing decisions are made by each user or at least dictated by each user so that information flows through the system to all users without being discriminated by a switching mode. However, in many applications the information flowing on the system may be of a sensitive nature and the fact that it is available to any and all users of the system can be rather disconserting. In these cases encryption and decryption is frequently employed to protect such information, but encryption and decryption can be an expensive process and leads to certain difficulties. Among these difficulties is providing conferencing interconnections, since all users must synchronize to the same encryption code simultaneously.
An alternative to encryption and decryption is a protected wireline system system where precautions are taken to prevent both physical access to the transmission path and electrical emanations from that path. Unfortunately, such transmission paths are usually expensive and employed only in point-to-point applications.
In point-to-point applications there are known three intrusion resistance systems which utilize optical fiber techniques that can be utilized for information protection. One such protection system employs an alarm technique that monitors the relative energy contained in mode groups emerging at high and low angles. Changes in the balance of the mode groups are construed as intrusion attempts and produces an alarm. A second protection system is an improvement of the first system where a subcarrier is injected at a 180.degree. phase difference in the two mode groups. A balance in the mode groups at the far end of the system results in the two outputs having no subcarrier frequency present in its output. Since the subcarrier is at a known frequency, integration techniques can be employed to improve the intrusion detection threshold. A third protection system uses a double core fiber structure developed by the assignee of the present invention. The intrusion resistance feature is obtained by transmitting the information on a central core of the double core fiber structure and transmitting a masking signal on an outer core of the double core fiber structure. The output level of the masking signal is monitored at the far end to betray intrusion attempts while the use of the masking signal forces the intruder to make a major disturbance in the fiber to extract the information in the central core. Such a disturbance has a higher detection probability.